The present invention relates to a fluid flow control valve which is actuated by the fluid proper. The valve comprises a valve housing formed with a seat for cooperating with a valve member linearly displaceable in the housing, a first and a second chamber located at either side of the valve seat and a differential or stepped piston rigidly connected to the valve member. The smaller surface of the piston is subjected to the fluid pressure prevailing in the first chamber while the opposed larger surface of the piston is subjected to a fluid pressure prevailing in a third chamber which via a switch valve may be connected selectively to the first or to the second chamber.
In a prior art valve of the kind referred to, switching of the switch valve from one position to the other results in a direct change in the pressure prevailing in the third chamber and hence a reversal of the direction of the resultant force exerted by the opposed fluid pressures acting on opposite sides of the stepped piston, which in turn causes either opening or closing of the flow control or main valve. Consequently, the switch valve functions as a pilot valve which permits the actuation of the main valve by means of relatively small actuating forces.